Process for the production of highpurity aromatic hydrocarbons



Nov. 13, 1945. D. F. GouLD PROCESS FOR THE PRODUCTION OF HIGH-PURITY AROMATIC HYDROCARBONS Filed Aug. 16, 1941 Sk KSS Patented oy. i3, i5

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TES i PATENT FFICE PROCESS FOR THE PRODUCTION F HIGH- PURITY AROMATIC HYDROCARBONS David F. Gould, Riverton, N. J., assignor, by mesne assignments, to Allied Chemical & Dye Corpo- 8 Claims.

This invention relates to the preparation of high-grade, relatively low-boiling aromatic hydrocarbons selected from the group consisting of unsubstituted aromatic hydrocarbons and aromatic hydrocarbons substituted in the nucleus by a saturated side chain, more particularly to the preparation of high-grade benzene.

As is well known, relatively low-boiling, aromatic hydrocarbons such as benzene, toluene and xylene are normally marketed in several grades. For example, the various grades of benzene are commonlyknown as Thiophene Free Benzol, Nitrat-.ion Pure Benzol, Industrial Pure Benzo and Industrial 90% Benzol, in order of decreasing purity. 'I'he purest grades of such lowboiling aromatic hydrocarbons are usually prepared by subjecting less pure grades to close fractionation and drastic acid-washing conditions in order to remove impurities therefrom, chiefly olens; acid washing under the drastic conditions necessary for the production of high-grade products in this manner causes substantial loss of the desired aromatic hydrocarbons by reaction with the acid, and the close fractionation involves relatively high operating costs and investment for equipment: hence, the processes for making the highest-grade aromatic hydrocarbons are relatively costly.

It is well known in the art that relatively lowboiling aromatic hydrocarbons of the type above described may be sulfonated by passing vapors thereof through sulfuric acid at elevated temperatures and in amounts such that the water generated by the reaction is swept out of the sulfonation mass by unsulfonated vapors passing therethrough; these unsulfonated hydrocarbon vapors, together with the admixed water vapor, are condensed, the water separated therefrom and the hydrocarbon thus recovered recycled through the sulfonator. It has been 'found that recycling of the hydrocarbon in this manner tends to build up an increasing content of paraiilnlc impurities which are not attacked by the sulfonating acid. Accordingly, as the hydrocarbon is recycled, increasing quantities of an inert substance must be vaporized and forced through the sulfuric acid. Hence, after the aromatic hydrocarbon has been recycled for a considerable time it becomes so diluted with inert paramnic hydrocarbons that the operation becomes highly ineihcient. When this stage is reached, the hydrocarbon mixture is no longer suitable for further sulfonation. I have found that the paraflins contained therein have boiling points so close to that of the aromatic hydrocarbon being treated (Cl. E-674) that separation of the aromatic hydrocarbon from the paraiiins by ordinary fractional distillation is impossible, so that this hydrocarbon mixture has a relatively low value.

It is an object of this invention to provide an economical process for the preparation of highgrade relatively low-boiling aromatic hydrocarbons selected from the group consisting of unsubstituted aromatic hydrocarbons and aromatic hydrocarbons substituted in the nucleus by a saturated side chain. v

It is another object of this invention to provide a method whereby parafn-containing aromatic hydrocarbons subjected to vapor phase sulfonation in the manner above described may be economically utilized.

I have made the discovery that relatively lowboiling aromatic hydrocarbons selected from the group consisting of unsubstituted aromatic hydrocarbons and aromatic hydrocarbons substituted in the nucleus by a saturated side chain' of the highest grade may be obtained from less pure grades thereof containing like-boiling parainic hydrocarbons in a highly economical manner by subjecting said less pure grades to sulfonation, removing the excess hydrocarbon from the sulfonating agent and subjecting the hydrocarbon to azeotropic distillation in the presence of an agent forming azeotropes with the likeboiling paraflinic hydrocarbons contained therein. The term like-boiling parailins is used throughout the specification and claims to refer to the paramns contained in the aromatic hydrocarbon which are not separable therefrom by ordinary fractional distillation. A preferred embodiment of my invention involves preparation of high-purity benzene by passing 1ess purebenzene through sulfuric acid at elevated temperatures, condensing the excess vapors emerging from the acid, separating the water therefrom, recycling the benzene through the sulfonator until the like-boiling paraiiin content of the condensed excess benzene vapors builds up to a point such that the recycling is no longer eilicient, preferably to not more than about 20% to about 25% of the mixture, then mixing this recycled benzene with an azeotropic agent forming minimum-boiling azeotropes with the like-boiling parainic hydrocarbons contained therein, and subjecting the mixture to azeotropic distillation. whereby the azeotropic mixture of the like-boiling paralilns and the agent is selectively distilled from the benzene, leaving high-grade product in the residue. I have found that by subjecting the less pure aromatic hydrocarbons to the process of my invention the non-Parafllnic impurities are removed by contact with the sulfuric acid, so that an aromatic hydrocarbon o! unusually high purity, except for its content of like-boiling parailins, is thereby obtained; hence, upon removal of these like-boiling paramns azeotropically, a product satisfying the most exacting standards of purity is produced. For example, when operating in accordance with the preferred embodiment of my invention, benzene satisfying or surpassing the specications of 'I'hiophene Free Benzol and "Nitration Pure Benaol may readily be produced.

The practice of my invention effects considerable. economies since the losses of aromatic hypoint of the hydrocarbon and preferably in the range of between about 150 and about 200 C. If desired, pressures somewhat below atmospheric may be employed in order to assist vaporization drocarbon inevitably incurred by subjecting such hydrocarbons to conventional drastic acid washes are not detrimental but represent conversion of the aromatic hydrocarbon into a sulfonlc acid of commercial utility; hence, reactions which ordinarily would result in losses, instead of representing an economic waste, as is the case in normal acid-washing procedures, are in my process utilized for the production of a valuable by-product. Furthermore, I have found that the removal of oleiins from the hydrocarbon in the sulfonation step yields a hydrocarbon from which the like-boiling paraiilns may be removed with surprising ease by azeotropic distillation. As a result, my invention provides a well integrated method for producing, on the one hand, valuable aromatic sulfonic acids such as benzene sulfonic acid and p-toluene sulfonic acid which may be converted to phenol and p-cresol respectively, andmon the other hand, aromatic hydrocarbons of highest degree of purity.

'Ijhe aromatic hydrocarbons most suitable for treatment in accordance with this invention may be any of the relatively low-boiling aromatic hydrocarbons selected from the group consisting of unsubstituted aromatic hydrocarbons and aromatic hydrocarbons substituted in the nucleus by a saturated side chain, the term relatively low-boiling meaning the boiling point is below about 150 C.; representatives of this class of hydrocarbons are benzene, toluene and the xylenes. Before being subjected to the sulfonation step of my invention such hydrocarbons may, if desired, be subjected to an acid wash carried out, for example, by washing the hydrocarbon three times with 1% of 66 B. sulfuric acid, whereby a large portion of the oleiinic impurities are removed; such a wash, however, leaves most of the paramnic impurities and may leave a small percentage of the oleilnic impurities in the hydrocarbon. The hydrocarbons to be treated may also be subjected to a preliminary fractionation in order to remove impurities having boiling points substantially different from that of-the desired hydrocarbon.

In carrying out the sulfonation step of my invention, the hydrocarbon to be treated may be vaporized in any suitable manner and then passed through sulfuric acid maintained at an elevated temperature in an amount such that excess hydrocarbon vapors passing from the acid remove the water generated by the sulfonation reaction. When operating in accordance with the preferred embodiment of my invention, i. e. when sulfonating benzene, the temperature of the sulfuric acid may vary widely, e. g. between the boiling point of benzene, i. e. 80 C., and about 200 C. When subjecting other hydrocarbons, e. g. toluene or xylenes, to this sulfonation step, the temperature should be above the boiling of the hydrocarbon and thereby permit lowering of the sulfonation temperature if advisable. The sulfonation may be carried out continuously or as a batch operation and the sulfonic acid may be withdrawn as desired. I

The excess hydrocarbon vapors passing from the sulfuric acid, together with the water vapor, may be condensed and the water vapor separated therefrom. The condensate thus obtained may desirably be recycled through the sulfuric acid in order to convert more of the hydrocarbon into the sulfonic acid, but may be subjected to the areotropic distillation step of my invention without recycling. Recycling of the hydrocarbon, however, causes the parailinic impurities to build up therein, with the result that the hydrocarbon tends to become unsuitable for further sulfonation; hence, the recycling of the hydrocarbon is preferably discontinued when its paramnic content has increased to not more than about 20% to about 25%. It is to be understood that the extent to which parailinic impurities are permitted to build up in the hydrocarbon depends on a variety of factors such as the sulfonating conditions employed, the particular product, i. e. sulfonic acid or pure hydrocarbon, desired, and the emciency of the distillation. For example, if it is desired to produce large quantities of high-purity benzene by this invention, the recycled benzene may be withdrawn from the sulfonation process and azeotropically distilled after its paraffin content has increased only slightly, e. g. to about 5%; on the other hand, if benzene sulfonic acid production is of paramount importance, the benzene may be recycled until its paramn content is much higher, e. g. 15% to 25% or more. These paramnic impurities remaining in the aromatic hydrocarbon may not be removed therefrom by conventional fractional distillation.

In accordance with my invention, the unsuifonated aromatic hydrocarbon obtained from the sulfonation step is mixed with an azeotropic agent which forms minimum-boiling azeotropes with the like-boiling paraillnic impurities present in the aromatic hydrocarbon and the mixture is then subjected to areotropic distillation, whereby azeotropic mixtures of the paraflins and the agent distill from the aromatic hydrocarbon, leaving a'substantially pure aromatic hydrocarbon in the residue. The agent employed Vmay be any agent forming minimum-boiling azeotropes with the like-boiling parafilnic constituents of the hydrocarbon mixture, which mixtures distill from the aromatic hydrocarbon. and thus may vary, depending upon the particular aromatic hydrocarbon being treated; the agent may also form an azeotrope with the aromatic hydrocarbon itself but this azeotrope, it has been found,

distills at a temperature substantially higher butyraldehyde, or ethyl formate could be em-l ployed, the use yoi.' acetone being preferred. When toluene is the aromatic hydrocarbon being treated, methanol, ethanol, n-propanol, isopropanol, n-butanol, n-amyl alcohol, tertiary amyi alcohol, crotonaldehyde, ethylene diamine, p-dioxane, acetic acid, acetonitrile, nitroeithne, 2-nitropropane, acrylonitrile, pyridine, ally a oub hol or a mixture of methylethyl ketone ant water may be employed as the azeotropic agen In case xylenes are being subjected to the prac ess of my invention, methanol. pyridin, a (yf alcohol, acetic acid, moncmethyl et e' d ethylene glycol, or morpholine may be emp oybe as the azeotropic agent. Other agents calname of forming minimum-boiling azeotropes wit e parafllnic constituents of the aromatic hydrocarbon being treated may also be employed. i t

In carrying out the distillation a suflic ent amount of the azeotropic agent should be miesen so that at some point in the rectification co umirlil, ordinarily at the top thereof, a temperature uri; be maintained which does not exceed the bo g point of the aromatic hydrocarbon being purified or the boiling point of the azeotropic mixture ci the aromatic hydrocarbon and azeotropic agen1 provided such an azeotrope is formed. The fo lowing tables list temperatures, referred to as control temperatures, which should not be exceeded in the column when subjecting the aromatic hydrocarbons mentioned to azeotropic distillation with the agents listed.

Control temperatures when distilling benzene Control tempeinture, C.

Control temperatures when distlling toluene Control tem- Agent peraturo, C.

Eesss Ulbl Control vtenmeratuns when distilliny :rulenes Control tem- Agent perature, C.

Allyl alcohol nu Pyr-id ine maintainin a temperature in the column beiv the tempegrature at which the aromatic hydrocarbon, or the azeotropic mixture of thte aromatic hydrocarbon and the azeotropic agen boils, the paraiilnic impurities are removed in the form of their azeotropic mixtures without substantial amounts of the desired aromatic hydrocarbons going over in the distillate. Furthermore, by controlling the temperature in this manner, the amount of azeotropic agent which should be present in the mixture being subjected to distillation may also be regulated since, if the temperature tends to rise above the control temperature, it is an indication that there is not a suiilcient amount of the agent present, with the to the distillation or the pa and possibly the aromatic hydrocarbon ing is given to the operator should be added in order azeotropic mixture. It will be found that the particular amount of azeotropic agent added will vary, depending upon the agent employed, upon the nature and amount of the paraiiinic impurities, and upon the nature oi' the aromatic hydrocarbon being distilled, but that by operating as above described the required amount of agent may always be maintained in the system.

After the azeotropic mixture of the paraiiinic impurities and the agent have been removed, the distillation may be discontinued, the desired aromatic hydrocarbon separated from the excess azeotropic agent. ii' any, in any suitable manner, e. g. by distillation or with selective solvents. The aromatic hydrocarbon thus obtained in every case 1will satisfy the most exacting standards of puri While a batch distillation has been speciilcally described, the distillation may be carried out continuously, if desired, by continuously forming a mixture of the agent and aromatic hydrocarbon to :be distilled, continuously separating the azeotropic mixture therefrom, and continuously withdrawing purified aromatic hydrocarbon from the mass being subjected to distillation.

A preferred embodiment of my invention is illustrated diagrammatically in Figure 1. As shown in Figure 1, benzene is vaporized in vaporizer I and the benzene vapors pass through a body oi sulfuric acid in sulfonator 2, the temperature of the acid being above the boiling point oi benzene. Benzene sulfonic acid is withdrawn from sulfonator 2 as produced. Unreacted benzene vapors emerging from sulfonator 2 are passed to condenser 3; the benzene condensate may, if desired, be returned, as shown, to vaporlzer I for further contact with sulfuric acid. Howevenin order to prevent undue build-up of parafilns in the benzene thus recycled, a portion of the benzene condensate is withdrawn from condenser 3 to still I, In still 4, the benzene containing like-boiling paraiiinic impurities is mixed with anagent forming minimum boilng azeotropes with the paraiiln impurities, and the resulting mixture distilled; the vapors evolved are condensed in condenser 5 and consist of a mixture of paraflin hydrocarbons and agent. The condensate from condenser 5 may be either discarded or returned to the upper plates of the column of still I, as desired. Substantially pure benzene is withdrawn from still I as residue.

The following example is illustrative of my invention. Amounts are given in parts by volume.

A batch of acid-washed benzene was subjected to sulfonation by vaporizing the benzene, passing it through sulfuric acid at a temperature between about 140 and about 200 C., condensing the excess benzene emerging from the acid, separating the benzene from the water condensed therewith, and recycling the. benzene through the acid until the parailin content thereof amounted to about 6%: the benzene sulfonic acid formed was recovered.

1,935 parts of the unsulfonated benzene recovered from the above sulfonation and containing 6% 'paraillns were mixed with 1.000 parts of acetone. and the mixture fractionally distilled in a still provided with a rectifying column, employing a 10 to 1 reflux ratio. Distillation began at 48.3 C. and quickly rose to 53.5 C. While itself; h nce, a warnthat additional agent to form the desired result that the distillation temperature rises due the paramn-acetone azeotrope was disumng over' the temperature in the column rose to 55.8 C. After the paramn-acetone azeotrope had been removed the temperature rose rapidly to the boiling point of benzene and a substantially pure benzene cut was obtained between the tempera- I scribed was also taken from the distillation. 890

parts of the cut were distilled in the presence of 495 parts of acetone, employing a reflux ratio of -20 to 1. Distillation began at 55.9 C., but quickly rose to the boiling point of benzene. A cut distilling between '79.9 and 80 C. was taken. Upon testing it was found this cut distilled over a range of 80.1 to 80.4 C., contained no paramns or sulfur compounds, and had an excellent color and odor; it specific gravity was 0.883 at 15.5 C.

From the above description it will be evident that the process of my invention provides an excellent method for preparing relatively lowboiling aromatic hydrocarbons of high degrees of purity. The production of such highly pure hydrocarbons has recently become increasingly important for precision work such as spectrographic or other optical work employing such hydrocarbons as solvents, since even the small amounts of parailinic impurities present in less Hence, it will undoubtedly be t0 those skilled in the art.

Since certain changes may be made in vcarrying out the above process without departing from I the scope of the invention, it is intended that t. cedure, this initial step could be carried out by subjecting the relatively impure low-boiling aromatic hydrocarbon, e. g. benzene, toluene or "j -xylene, to liquid phase sulfonation by treating it '.f with an amount of sulfuric acid less than that required to completely sulfonate the hydrocarbon, and the excess hydrocarbon then subjected to azectropic distillation in accordance with this invention.

I claim: v

l. A process for the production of a high-purity relatively low-boiling aromatic hydrocarbon and the corresponding sulfonic acid, said hydrocarbon being selected from the group consisting of relatively low-boiling unsubstituted aromatic hydrocarbons and relatively low-boiling aromatic hydrocarbons substituted in the nucleus by a saturated side chain, which process comprises subjecting vapors of a less pure, relatively lowboiling aromatic hydrocarbon selected from the above group containing like-boiling parailinic hydrocarbons to contact with a body of liquid sulfuric acid of sulfonating strength at a temperature not below the boiling point of the hydrocarbon at the pressure employed, withdrawing the aromatic hydrocarbon sulfonic acid .thus produced, condensing unreacted aromatic hydrocarbon vapors from the sulfuric acid. sepapure grades such as Industrial Pure Benzol,'l

' above referred to, is often undesirable. Further- 'j' more, it provides a process for the economical Y utilization of recycled aromatic hydrocarbons produced in vapor phase sulfonation processes. of greatlnterest rating water from the condensate. vaporiling the aromatic hydrocarbon condensate thus separated. recycling said vapors through the sulfuric acid. continuing condensation of excess vapors passing from said acid. separation of water, vaporisation and recycling until the paramn content of the aromatic hydrocarbon bein MGM build! UP substantially, and then subjecting this recycled aromatic hydrocarbon containing like-boiling paramnic hydrocarbons to aseotropic distillation in the presence of an agent forming minimum boiling azeotropes with the paramnlc hydrocarbons contained therein to produce the high-purity aromatic hydrocarbon.

2. A process for the production of a high-purity relatively low-boiling'aromatic hydrocarbon and the corresponding sulfonic acid, said hydrocarbon being selected from the group consisting of relatively low-boiling unsubstituted aromatic hydrocarbons and relatively low-boiling aromatic hydrocarbons substituted in the nuclem by l. saturated side chain, which process comprises subjecting vapors of a less pure, relatively lowboiling aromatic hydrocarbon selected from the above group containing like-boiling parailinic hydrocarbons to contact with a body of liquid sulfuric acid of sulfonatins strength at a temperature not below the boiling point of the hydrocarbon at the pressure employed. withdrawing the aromatic hydrocarbon sulfonic acid thus produced, condensing unreacted hydrocarbon vapors psing from the sulfuric acid, separating water from the condensate, vaporlzing the aromatic hydrocarbon condensate thus separated, recycling said vapors through the sulfuric acid. continuing condensation of excess vapors passing from said acid, separation of water, vaporlsation and recycling until the parailiii' contentof the aromatic hydrocarbon being recycled builds up to not more than from about 20% to about 25% thereof, and then subjecting this recycled aromatic hydrocarbon containing like-boiling paraiiinic hydrocarbons to azeotropic distillation in the presence of an agent forming minimum boiling aseotropes with the parailinic hydrocarbons contained therein to produce the high-purity aromatic hydrocarbon.

3. A process for preparing thiophene-free benzene and benzene sulfonic acid. which comprises passing vapors of acid-washed crude benzene cmtaining like-boiling paramnic hydrocarbons through a body of liquid sulfuric acid of sulfonsting strength at a temperature not below the boiling point of the benzene at thepressure employed, withdrawing the benzene sulfonic acid thus produced, condensing unrcacted benzene vapors emerging from the acid, separating water from the condensate, vaporlzing the benzene thus separated, recycling the benzene vapors through the sulfuric acid. continuing condensation of excess vapors passing from the acid. separation of water, vaporization andrecycling until the paramn content of the benzene thus recycled builds up to not more than about 20% to about 25% thereof, then mixing the benzene thus recycled containing like-boiling parafiinic hydrocarbons with an aseotropic agent forming minimum boiling azeotropes with the like-boiling paramnic hydrocarbons contained therein, subjecting the mixture to aseotropic distillation until the paramos have been substantially completely removed, and recovering a substantially pure thiophene-free benzene product.

4- A prwess for preparing thiophme-free bensene and benzene sulfonic acid. which ccmprlscs passing vapors of acid-washed, crude benzene containing like-boiling parailinic hydrocarbons through a body of liquid sulfuric acid of sulfonating strength at a temperature not below the boiling point of the benzene at the pressure employed, withdrawing the benzene sulfonic acid thus produced, condensing unreacted benzene vapors emerging from the acid, separating water from the condensate, vaporizing the benzene thus separated, recycling the benzene vapors through the sulfuric acid, continuing condensation of excess vapors passing from the acid, separation of water, vaporization and recycling until the paraiiin content of the benzene thus recycled builds up to not more than about 20% to about 25% thereof, then mixing the benzene thus recycled containing like-boiling paraillnic hydrocarbon with acetone, subjecting the mixture to azeotropic distillation until the like-boiling paraiilns contained therein have Abeen substantially completely removed in the form of an azeotropic mixture with acetone, and recovering a substantially pure thiophene-free benzene product.

5. A process for preparing thiophene-free benzene and benzene sulfonic acid, which comprises passing vapors of acid-washed, crude benzene containing like-boiling parainic hydrocarbons through a body of liquid sulfuric acid of sulfonating strength at a temperature not below the boiling point of the benzene at the pressure employed, withdrawing the benzene sulfonic acid thus produced, condensing unreacted benzene vapors emerging from the acid, separating water from the condensate, vaporizing the benzene thus separated, recycling the benzene vapors through the sulfuric acid, continuing condensation of excess vapors passing from the acid, separation of water, vaporization and recycling until the parailin content of the benzene thus recycled builds up to not more than about 20% to about 25% thereof, then mixing the benzene thus recycled containing like-boiling parailinic hydrocarbons with methanol, subjecting the mixture to azeotropic distillation until the like-boiling paraiiins contained therein have been substantially completely removed in the form of an azeotropic mixture with methanol, and recovering a substantially pure thiophene-free benzene product.

6. A process for preparing high-purity toluene and toluene sulfonic acid, which comprises passing vapors of acid-washed, crude toluene containing like-boiling paramnic hydrocarbons through a body of liquid sulfuric acid of sulfonating strength at a temperature not below the boiling point of the toluene at the pressure employed,

withdrawing the toluene sulfonic acid thus produced, condensing unreacted toluene vapors emerging from the acid, separating Water from the condensate, vaporizing the toluene thus separated, recycling the toluene vapors through the sulfuric acid, continuing condensation of excess toluene vapors passing from the acid, separation of water, vaporization and recycling until the parailin content of the toluene being recycled builds up to not more than about 20% to about 25% thereof, then mixing the toluene thus recycled containing like-boiling parailinic hydrocarbons with an azeotropic agent forming minimum boiling azeotropes with the like-boilingparaiiinic hydrocarbons contained therein, subjecting the mixture to azeotropic distillation until the parailins have been substantially completely removed, and recovering a substantially pure toluene product.

7. A process for preparing high-purity toluene and toluene sulfonic acid, which comprises passing vapors of acid-washed, crude toluene containing like boiling paramnic hydrocarbons through a body of liquid sulfuric acid of sulfonating strength at a temperature not below the boilings point of toluene at the pressure employed, withdrawing the toluene sulfonic acid thus produced, condensing unreacted toluene vapors emerging from the acid, separating water from the condensate, vaporizing the toluene thus separated, recycling the toluene vapors through the sulfuric acid, continuing condensation of excess vapors passing from the acid, separation of water, vaporization and recycling until the paraiiin content of the toluene being recycled builds up to not more than about 20% to about 25% thereof, then mixing the toluene thus recycled containing like-boiling parafiinic hydrocarbons with methanol, subjecting the mixture to azeotropic distillation until the like-boiling paraliins contained therein have been substantially completely removed in the form of an azeotropic mixture with methanol, and recovering a, substantially pure toluene product.

8. A process for the production of a highpurity relatively low-boiling aromatic hydrocarbon and the corresponding sulfonic acid, said hydrocarbon being selected from the group consisting of relatively low-boiling unsubstituted aromatic hydrocarbons and relatively low-boiling aromatic hydrocarbons substituted in the nucleus by a saturated side chain, which process comprises subjecting vapors of a less pure, relatively low-boiling aromatic hydrocarbon selected from the above group containing like-boiling paralnic hydrocarbons to contact with a body of liquid sulfuric acid of sulfonating strength at a temperature not below the -boiling point of the hydrocarbon at the pressure employed, withdrawing the aromatic hydrocarbon sulfonic acid thus produced, condensing unreacted aromatic hydrocarbon vapors passing from the sulfuric acid, separating water from the condensate, vaporizing the aromatic hydrocarbon condensate thus separated, recycling said vapors through the sulfuric acid, continuing condensation of excess vapors passing from said acid, separation of water, vaporization and recycling until the parailin content of the aromatic hydrocarbon being recycled builds up substantially, and then subjecting'this recycled aromatic hydrocarbon containing likeboiling paraiiinic hydrocarbons to azeotropic distillation in the presence of an agent forming azeotropes with the parailinic hydrocarbons contained therein to produce the high-purity aromatic hydrocarbon.

i DAVID F. GOULD. 

